For the LTE-Advanced, carrier aggregation (CA: Carrier Aggregation) has been adopted in which communication is performed by simultaneously using a plurality of carriers while using bandwidths supported by the LTE (up to 20 MHz) as basic units, so that throughput can be achieved that exceeds that of the LTE, while maintaining backward compatibility with the LTE (cf. Non-Patent Document 1, for example). A carrier that can be a basic unit in the carrier aggregation is referred to as a component carrier (CC: Component Carrier).
When the CA is performed, a PCell (Primary cell), which is a highly reliable cell for ensuring connectivity, and a SCell (Secondary cell), which is a subsidiary cell, are established for user equipment UE. First, the user equipment UE connects to the PCell; and can add the SCell, depending on necessity. The PCell is the same as a cell of the LTE scheme that supports the RLM (Radio Link Monitoring), the SPS (Semi-Persistent Scheduling), and so forth.
The SCell is established for the user equipment UE, in addition to the PCell. Addition and deletion of the SCell are performed through the RRC (Radio Resource Control) signaling. The SCell is in a deactivated state (deactivate state) immediately after it is established for the user equipment UE, so that it is a cell in which communication is enabled (scheduling is enabled) only after it is activated in the MAC (Media Access Control) layer. For the SCell, activation/deactivation is controlled by a MAC signal from the base station eNB.
Note that, for TDD, the PCell is formed of a single CC; and for FDD, the PCell is formed of an uplink CC and a downlink CC. Further, for TDD, the SCell is formed of a single CC; and for FDD, the SCell is formed of a downlink CC (and an uplink CC, depending on necessity). Furthermore, there are cases where the “cell,” such as the PCell and the SCell, is used as a synonym of the CC.